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For one, semen contains a hormone-like substance called prostaglandin, which is used in synthetic form to induce labour. It was also suggested that orgasms could trigger uterine contractions.

However, few studies have ever investigated whether sex really can start the birthing process and the small amount of existing evidence has been inconclusive.

In the latest study, published in the journal BJOG, the team set out to see whether advising women to have sex during the final weeks of pregnancy could help them avoid an induction.

'Labour induction for prolonged pregnancy is common and many women are also tempted for a variety of personal reasons to trigger labor off in the very latter stages of pregnancy,' Dr Chiong said.

Study confirmed sex late on in pregnancy is usually safe, even if it doesn't speed up labour

The researchers invited more than 1,100 women to participate, all of whom were 35 to 38 weeks pregnant (the typical pregnancy lasts 40 weeks) and none of whom had had sex in the previous six weeks.

Roughly half of the women were advised by a doctor to have sex frequently as a means of safely expediting labour.

The other half were told that sex was safe during pregnancy, but that its effects on labour were unknown.

The researchers then tracked the women to determine how long their pregnancies lasted and whether they required any medical intervention to start labour.

They found that about 85 percent of the women who were encouraged to have sex did follow the doctor's advice, while 80 per cent of women in the other group also had sex.

Women in the group advised to have sex also had it more frequently for the remaining duration of their pregnancies - three times versus two.

But the rates of induced labor were similar in both groups: 22 per cent of those advised to have sex and 20.8 per cent of the other group had inductions, a difference so small it is likely to have been driven by chance.

Pregnancy also lasted an average of 39 weeks for both groups.

Dr Jonathan Schaffir at The Ohio State University College of Medicine who has studied obstetrical folklore, said the findings offer good support for the idea that sex won't induce labour.

Earlier research had relied primarily on surveys of women about their sexual experiences during pregnancy, but this study was 'the first attempt to really randomise the experience, for some to have sex and some to not, which is a very hard thing to do,' he said.

Schaffir wouldn't advise his patients to have sex for the express purpose of going into labour, he added, but the study did indicate that there aren't any hazards to it.

'Even though this study did not show any increase in the rate of labour or a decrease in the rate of induction, it helped to cement the idea that having sex is probably safe if you want it,' he said.

Dr Chiang Tan said the results show that pregnancy evolved to be resistant to disruption.

'Human pregnancy has to be robust to a little adventure like intercourse and unfortunately for our purpose, it seems pretty robust to the very end,' he said.